ci: Cache pantry (hackage metadata) separately
ci: Cache pantry (hackage metadata) separately

This further reduces our usage of cache storage by caching the metadata
only once per repo. Now the metadata cache is ~250MB and the individual
caches with built dependencies are ~5M each.

Related: https://github.com/xmonad/X11/pull/75
Related: https://github.com/xmonad/xmonad/issues/283

xmonad/xmonad